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<a name="Modifiers"></a>
<div class="header">
<p>
Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Class-Preferences.html#Class-Preferences" accesskey="p" rel="prev">Class Preferences</a>, Up: <a href="Constraints.html#Constraints" accesskey="u" rel="up">Constraints</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Constraint-Modifier-Characters"></a>
<h4 class="subsection">17.8.4 Constraint Modifier Characters</h4>
<a name="index-modifiers-in-constraints"></a>
<a name="index-constraint-modifier-characters"></a>

<p>Here are constraint modifier characters.
</p>
<dl compact="compact">
<dd><a name="index-_003d-in-constraint"></a>
</dd>
<dt>&lsquo;<samp>=</samp>&rsquo;</dt>
<dd><p>Means that this operand is written to by this instruction:
the previous value is discarded and replaced by new data.
</p>
<a name="index-_002b-in-constraint"></a>
</dd>
<dt>&lsquo;<samp>+</samp>&rsquo;</dt>
<dd><p>Means that this operand is both read and written by the instruction.
</p>
<p>When the compiler fixes up the operands to satisfy the constraints,
it needs to know which operands are read by the instruction and
which are written by it.  &lsquo;<samp>=</samp>&rsquo; identifies an operand which is only
written; &lsquo;<samp>+</samp>&rsquo; identifies an operand that is both read and written; all
other operands are assumed to only be read.
</p>
<p>If you specify &lsquo;<samp>=</samp>&rsquo; or &lsquo;<samp>+</samp>&rsquo; in a constraint, you put it in the
first character of the constraint string.
</p>
<a name="index-_0026-in-constraint"></a>
<a name="index-earlyclobber-operand"></a>
</dd>
<dt>&lsquo;<samp>&amp;</samp>&rsquo;</dt>
<dd><p>Means (in a particular alternative) that this operand is an
<em>earlyclobber</em> operand, which is written before the instruction is
finished using the input operands.  Therefore, this operand may not lie
in a register that is read by the instruction or as part of any memory
address.
</p>
<p>&lsquo;<samp>&amp;</samp>&rsquo; applies only to the alternative in which it is written.  In
constraints with multiple alternatives, sometimes one alternative
requires &lsquo;<samp>&amp;</samp>&rsquo; while others do not.  See, for example, the
&lsquo;<samp>movdf</samp>&rsquo; insn of the 68000.
</p>
<p>A operand which is read by the instruction can be tied to an earlyclobber
operand if its only use as an input occurs before the early result is
written.  Adding alternatives of this form often allows GCC to produce
better code when only some of the read operands can be affected by the
earlyclobber. See, for example, the &lsquo;<samp>mulsi3</samp>&rsquo; insn of the ARM.
</p>
<p>Furthermore, if the <em>earlyclobber</em> operand is also a read/write
operand, then that operand is written only after it&rsquo;s used.
</p>
<p>&lsquo;<samp>&amp;</samp>&rsquo; does not obviate the need to write &lsquo;<samp>=</samp>&rsquo; or &lsquo;<samp>+</samp>&rsquo;.  As
<em>earlyclobber</em> operands are always written, a read-only
<em>earlyclobber</em> operand is ill-formed and will be rejected by the
compiler.
</p>
<a name="index-_0025-in-constraint"></a>
</dd>
<dt>&lsquo;<samp>%</samp>&rsquo;</dt>
<dd><p>Declares the instruction to be commutative for this operand and the
following operand.  This means that the compiler may interchange the
two operands if that is the cheapest way to make all operands fit the
constraints.  &lsquo;<samp>%</samp>&rsquo; applies to all alternatives and must appear as
the first character in the constraint.  Only read-only operands can use
&lsquo;<samp>%</samp>&rsquo;.
</p>
<p>This is often used in patterns for addition instructions
that really have only two operands: the result must go in one of the
arguments.  Here for example, is how the 68000 halfword-add
instruction is defined:
</p>
<div class="smallexample">
<pre class="smallexample">(define_insn &quot;addhi3&quot;
  [(set (match_operand:HI 0 &quot;general_operand&quot; &quot;=m,r&quot;)
     (plus:HI (match_operand:HI 1 &quot;general_operand&quot; &quot;%0,0&quot;)
              (match_operand:HI 2 &quot;general_operand&quot; &quot;di,g&quot;)))]
  &hellip;)
</pre></div>
<p>GCC can only handle one commutative pair in an asm; if you use more,
the compiler may fail.  Note that you need not use the modifier if
the two alternatives are strictly identical; this would only waste
time in the reload pass.
The modifier is not operational after
register allocation, so the result of <code>define_peephole2</code>
and <code>define_split</code>s performed after reload cannot rely on
&lsquo;<samp>%</samp>&rsquo; to make the intended insn match.
</p>
<a name="index-_0023-in-constraint"></a>
</dd>
<dt>&lsquo;<samp>#</samp>&rsquo;</dt>
<dd><p>Says that all following characters, up to the next comma, are to be
ignored as a constraint.  They are significant only for choosing
register preferences.
</p>
<a name="index-_002a-in-constraint"></a>
</dd>
<dt>&lsquo;<samp>*</samp>&rsquo;</dt>
<dd><p>Says that the following character should be ignored when choosing
register preferences.  &lsquo;<samp>*</samp>&rsquo; has no effect on the meaning of the
constraint as a constraint, and no effect on reloading.  For LRA
&lsquo;<samp>*</samp>&rsquo; additionally disparages slightly the alternative if the
following character matches the operand.
</p>
<p>Here is an example: the 68000 has an instruction to sign-extend a
halfword in a data register, and can also sign-extend a value by
copying it into an address register.  While either kind of register is
acceptable, the constraints on an address-register destination are
less strict, so it is best if register allocation makes an address
register its goal.  Therefore, &lsquo;<samp>*</samp>&rsquo; is used so that the &lsquo;<samp>d</samp>&rsquo;
constraint letter (for data register) is ignored when computing
register preferences.
</p>
<div class="smallexample">
<pre class="smallexample">(define_insn &quot;extendhisi2&quot;
  [(set (match_operand:SI 0 &quot;general_operand&quot; &quot;=*d,a&quot;)
        (sign_extend:SI
         (match_operand:HI 1 &quot;general_operand&quot; &quot;0,g&quot;)))]
  &hellip;)
</pre></div>
</dd>
</dl>

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